Climate Outlook

Pacific Islands April - September 2002

Issued: March 2002

The IRI has prepared this experimental Climate Outlook for the Pacific Islands
for April - September 2002. Of relevance in the preparation of this
outlook is the prediction of eastern equatorial Pacific sea surface
temperatures (SSTs) warming to slightly above normal at the beginning of
the first forecast period. In the coming 6 months, the SSTs anomalies
are expected to increase somewhat more. Currently the SSTs
across much of the eastern and central equatorial are slightly above
their long-term average
(SSTs).
Higher than average SSTs have been in place in the far western Pacific
and near the international dateline over the last several months. Higher
than average SSTs have also more recently developed immediately off the west
coast of South America. This same general pattern is predicted to be
maintained in the first season of the forecast
April-June 2002, while during the second, third and fourth seasons
of the forecast the equatorial SST thoughout the entire eastern and central Pacific is predicted to become progressively warmer, attaining the level of a weak
El Nino (SSTs between 0.5 and 1 degree C above normal) by the fourth period
May-July 2002,
June-August 2002,
July-September 2002.
Note that this prediction for a weak El Nino is that given by one ENSO
prediction model. It is described here because it served as the boundary
forcing for the following climate prediction, which is primarily based
on dynamical global climate models. This should not be confused
with the IRI's ENSO Statement, which while indicating a significant
potential for an El Nino to develop also highlights the uncertainty still
present in the prediction at this time.
In the other tropical oceans, warmer than average SSTs continue to dominate
much of the Indian Ocean. These are expected to decrease slowly through the
forecast period, becoming slightly below normal near the coast of southern Asia
and in parts of western Indonesia in the later forecast periods.
The area of above-average temperature in the tropical north Atlantic Ocean is
expected to persist but gradually weaken through the four forecast periods.

METHODS -

This Outlook was prepared using the following
procedures and information:

A) Coupled ocean-atmosphere model predictions of tropical Pacific SST
covering the forecast period. Particularly heavy weighting has been
given to predictions from the coupled model operated by the NOAA
National Centers for Environmental Prediction, Climate Modeling Branch.
This model suggests a continuation of near-average conditions during the
first forecast season. The forecast for near-neutral conditions is
consistent with some, but not all, numerical and statistical forecasts
of central and eastern Pacific SSTs.

B) Forecasts of the tropical Indian ocean using a statistical model
developed by the IRI.

C) Global atmospheric general circulation model (GCM) predictions of the
atmospheric response to the present and predicted sea-surface
temperature patterns.

The procedures, models, and data used to derive this Climate Outlook may
be somewhat different from those used by the national meteorological
services in the region. Thus, this product may differ from the official
forecasts issued in those areas. The Climate Outlook for
April - September 2002
is dependent on the accuracy of the SST predictions. For the
tropical Pacific, these predictions can be expected to provide useful
information, but there is some uncertainty concerning the evolution of
SSTs. Spread (variation) in global SST predictions is a source of
uncertainty in the Outlook provided here. In particular, the forecasts
for the tropical Indian and Atlantic oceans have been an important
influence on the forecasts over Africa. Note that even if perfectly
accurate SST forecasts were possible, there would still be uncertainty
in the climate forecast due to chaotic internal variability of the
atmosphere. These uncertainties are reflected in the probabilities given
in the forecast.

It is stressed that the current status of seasonal-to-interannual
climate forecasting allows prediction of spatial and temporal averages,
and does not fully account for all factors that influence regional and
national climate variability. This Outlook is relevant only to seasonal
time scales and relatively large areas; local variations should be
expected, and variations within the 3-month period should also be
expected. For further information concerning this and other guidance
products, users are strongly advised to contact their National
Meteorological Services.

OUTLOOK -

This Outlook covers four seasons: April-June 2002,
May-July 2002, June-August 2002 and July-September 2002.
Maps are given showing tercile probabilities of
precipitation and temperature. The maps for precipitation indicate the
probabilities that the seasonal precipitation will fall into the wettest
third of the years (top number), the middle third of the years (middle
number), or the driest third of the years (bottom number). The color
shading indicates the probability of the most dominant tercile -- that
is, the tercile having the highest forecast probability. The color bar
alongside the map defines these dominant tercile probability levels. The
upper side of the color bar shows the colors used for increasingly
strong probabilities when the dominant tercile is the above-normal
tercile, while the lower side shows likewise for the below-normal
tercile. The gray color indicates an enhanced probability for the
near-normal tercile (nearly always limited to 40%). As before, numbers
and their associated histograms show the probabilities of the three
terciles. In areas with lots of spatial detail, there may not be
sufficient room on the map, to allow histograms for each region. In
those cases, some idea of the probabilities may be gained from the color
alone. A qualitative outlook of climatology ("C") indicates that there
is no basis for favoring any particular category.
Areas that are marked by "D" represent regions for which less than 3cm of
precipitation typically occurs over the season.
Otherwise, for example, in the case of
northern New Caledonia in April-June 2002
(Map A),
there is a 40% probability that the precipitation will be in the
wettest third of the years, a 35% chance it will be in the near-normal
third of the years, and a 25% chance that the precipitation will be in
the driest third of the years.

Maps of temperature show expected probabilities that the seasonal
temperatures will fall into the warmest third of the years, the middle
third of the years, or the coldest third of the years
(Map A).
The numbers for each region on the temperature maps
indicate the probabilities of temperatures to fall in each of the three
categories, above-, near-, and below-normal.

An
additional precipitation map
is provided for the first season indicating probabilities for extreme
precipitation anomalies. Extremes are defined as anomalies that fall
within the top and bottom 15th percentile of the observed records. A
priori, there is a 15% probability of being within the extremely wet
category, and a 15% probability of being within the extremely dry
category, leaving a 70% probability that the precipitation will not be
extreme. The maps indicate areas of increased risk of extreme
precipitation totals. Three levels of increased risk are defined:
slightly enhanced risk, enhanced risk, and greatly enhanced risk. For
slightly enhanced risk, there is a 25-40% probability that precipitation
will be within the indicated extreme, i.e. wet or dry. This represents
an approximate doubling of the climatological risk. For enhanced risk,
there is a 40-50% probability that precipitation will be within the
indicated extreme. This represents an approximate tripling of the
climatological risk. For greatly enhanced risk, the probability that
precipitation will be within the indicated extreme exceeds 50%, i.e. the
indicated extreme is the most likely outcome. A similar map is provided
in the first season indicating probabilities of
extreme temperature
anomalies.

Boundaries between sub-regions should be considered as transition zones,
and their location considered to be only qualitatively correct.

April-June 2002 through July-September 2002:

The following discussion briefly describes the probability anomaly forecasts:

Precipitation

Enhanced probabilities for below normal precipitation are forecast
for most of French Polynesia and vicinity during the first, third and
fourth forecast periods, and for near normal during the second forecast
period. This is the case because, despite the predicted development of
a weak El Nino during the coming few months, a west-to-east-elongated
area of slightly below normal SST is expected to prevail in the eastern
part of the Pacific Basin south of the equator, as the area along the
equator warms. Hawaii is also forecast to have slightly enhanced
probabilities for below normal rainfall during the first two forecast
periods. Most of the remainder of the Pacific Islands, including islands
near the equator from east of the date line westward, as well as islands
north and south of the equator west of the dateline, are forecast to have
enhanced probabilities for above normal precipitation in all four forecast
periods.

Temperature
An enhanced probability for above normal temperature is forecasted
for most of the tropical Pacific islands for all four forecast periods,
as a weak El Nino is forecast to develop during the next few months.
Exceptions are Hawaii during the first two forecast periods, eastern
Kiribati during the first period, and mainly the southern portion of
French Polynesia during the third and fourth periods. Those exceptions
have a slightly enhanced probability for below normal temperature in
response to expected SSTs slightly below normal.